The standard definition of reverse harem is when one woman has a relationship with three or more men.
The HLV virus wiped out 90% of the population, leaving the survivors to figure out how to keep surviving. They start to notice more men than women survived, which leads to polyamorous relationships.
More in the Hell Virus series (coming throughout 2019-2020):
BriarEdge Serial (to be combined in a boxed set upon completion):